In an internal combustion engine incorporated in a vehicle, sometimes such a phenomenon occurs that air-fuel mixture in a combustion chamber is not ignited depending on conditions such as temperature or the state of a spark plug, i.e., what is called a “misfire”. When a misfire occurs, not only the rotation speed of the engine is reduced, but also unburned air-fuel mixture is discharged to an exhaust manifold. This may degrade emission and may disadvantageously affect an emission purifying catalyst. Accordingly, in order for the misfire trouble in the internal combustion engine to be addressed at an early stage, occurrence of the misfire must be detected and reported to the driver.
As a misfire detection apparatus detecting a misfire in a multicylinder internal combustion engine, focusing on the fact that fluctuations in the rotation speed of the engine (hereinafter simply referred to as the rotation fluctuation) is great when a misfire has occurred, there has been proposed a misfire determination apparatus that determines occurrence of a misfire based on the rotation fluctuation, (for example, see Japanese Patent Laying-Open No. 05-203539 and Japanese Patent No. 3063007). The basic principle of misfire determination in such a misfire detection apparatus is as follows.
First, when a misfire occurs in one cylinder, the engine rotation speed in an explosion stroke (actually the misfire has occurred and therefore explosion has not occurred) in the cylinder is gradually reduced. As a result, the time period required for the crankshaft to rotate by a certain crank angle in the explosion stroke of the misfiring cylinder becomes longer than in an explosion stroke of the other cylinders. Accordingly, by measuring the time periods for the cylinders and comparing with each other, whether or not a misfire has occurred can be detected.
For example, as shown in FIG. 5, as to a six-cylinder internal combustion engine, when a certain cylinder (for example a third cylinder #3) is in an explosion stroke, an operation is performed to obtain a difference (T3−T2) between an elapsed time T3 required for the crankshaft to rotate by a certain crank angle in that explosion stroke, and an elapsed time T2 required for the crankshaft to rotate by a certain angle in an explosion stroke of a cylinder (for example a second cylinder #2) having been entered the explosion stroke a certain crank angle prior to the explosion stroke of third cylinder #3. When the operation value (the deviation of the elapsed time), that is, a rotation fluctuation amount, exceeds a prescribed threshold value, it is determined that a one-cylinder misfire (for example, a misfire of third cylinder #3) has occurred in the six-cylinder internal combustion engine. Further, as shown in FIG. 6, when rotation fluctuation amounts consecutively exceed a threshold value, it is determined that two-cylinder-consecutive misfires (for example, consecutive misfires of third cylinder #3 and fourth cylinder #4) have occurred.
It is noted that the rotation speed of the internal combustion engine is detected by a rotation angle sensor (for example a crank position sensor) provided at the crankshaft.
Meanwhile, in an internal combustion engine, rotation fluctuation occurs by a misfire, and in some cases by backlash due to deterioration with age of a flywheel or the like. In some cases, when a vehicle is traveling on a rough road (such as a bumpy road or a gravelly road), torque from the rear wheels of the vehicle is transmitted through the propeller shaft, whereby rotation fluctuation occurs in the flywheel. The rotation fluctuation due to a factor other than a misfire is hardly distinguished from the rotation fluctuation due to a misfire, and therefore a misfire may erroneously be detected.
Accordingly, in order to avoid such erroneous detection of a misfire, there has been proposed a technique of canceling misfire determination based on the presumption that there is backlash due to deterioration with age of a flywheel or the like when the average correlation between a rotation fluctuation value and a misfire determination value is great (for example, see Japanese Patent Laying-Open No. 2004-324524). Further, there has been proposed a technique of prohibiting misfire detection by determining that the vehicle is traveling on a rough road based on rotation fluctuation of the flywheel (for example, see Japanese Patent Laying-Open No. 07-279734).
As a flywheel attached to an internal combustion engine (hereinafter also referred to as an engine), a flywheel damper is known, which absorbs rotation fluctuation of the engine and/or torsional vibration of the rotation shaft to suppress vibration of the powertrain. However, an engine employing a flywheel damper has a problem that the rotation fluctuation becomes irregular when a misfire has occurred and reliability of misfire detection is impaired. The reason thereof is provided in the following.
First, the flywheel damper is a mechanism, for example as shown in FIG. 7, in which an engine-side plate 21 (hereinafter referred to a front plate 21) coupled to the crankshaft of the engine and a mission-side plate 22 (hereinafter referred to as a rear plate 22) coupled to the transmission side are provided, with a spring 23 being arranged between front plate 21 and rear plate 22. In such an engine employing such flywheel damper 2 also, the engine rotation speed is detected by a rotation angle sensor (for example, a crank position sensor) provided at the crankshaft.
In this type of flywheel damper 2, backlash (play: for example 6° (±3°)) is provided between rear plate 22 and spring 23 so that small rotation fluctuation in normal idling operation (with low load/low rotation speed) is absorbed by the backlash. Thus, as indicated by arrow B in FIG. 7, a state in which spring 23 contacts none of front plate 21 and rear plate 22 is maintained. In this state, though slight rotation fluctuation (for example fluctuation in a range of 800-810 rpm) occurs in front plate 21, rear plate 22 rotates substantially constantly (for example, 805 rpm). In flywheel damper 2, when spring 23 does not contact rear plate 22, motive power is transmitted by a frictional mechanism (not shown) or the like.
When a one-cylinder misfire has occurred in the above-described idling operation state, in some cases rear plate 22 moves in the above-described range of backlash, whereby the state in which spring 23 does not contact front plate 21 or rear plate 22 is maintained. Here, erroneous misfire detection may not occur.
On the other hand, when two-cylinder-consecutive misfires have occurred, as shown in FIG. 8, a rotation fluctuation amount due to misfire exceeds a threshold value in the first of the misfiring cylinders (for example the third cylinder) and a misfire is detected, based on the above-described reason. Then, when a misfire has consecutively occurred in the second of the misfiring cylinders (for example the fourth cylinder), the rotation speed of front plate 21 (engine rotation speed) becomes lower than the rotation speed of rear plate 22 (the rotation speed of the transmission side) (for example, front plate 21 being reduced to 790 rpm thus becoming lower than rear plate 22 being 805 rpm), whereby rear plate 22 contact one end 23a of spring 23.
This contact between rear plate 22 and one end 23a of spring 23 causes energy to be transmitted between rear plate 22 and front plate 21 via spring 23 (the energy of rear plate 22 being provided to front plate 21), whereby the reduction in the rotation speed of front plate 21 due to the misfire becomes small and the rotation fluctuation becomes small. As a result, the rotation fluctuation amount due to the misfire in the second of the misfiring cylinders does not exceed a threshold value. Thus, the misfire in the second of the consecutively misfiring cylinders (for example, the fourth cylinder) cannot be detected. It is noted that, not only the two-consecutive misfires, but also a one-cylinder misfire cannot be detected in some cases, depending on the size of backlash of flywheel damper 2.
When rear plate 22 contacts one end 23a of spring 23, the transmission of energy reduces the rotation speed of rear plate 22. At the time point where the rotation speed of rear plate 22 becomes smaller than the rotation speed of front plate 21, rear plate 22 is disengaged from one end 23a of spring 23, thereby falling within the above described range of backlash. Thereafter, when the rotation speed of front plate 21 becomes higher than that of rear plate 22 by normal combustion of the cylinder, rear plate 22 contacts the other end 23b of spring 23.
The contact between rear plate 22 and the other end 23b of spring 23 causes energy to be transmitted between rear plate 22 and front plate 21 via spring 23 (the energy of front plate 21 being provided to rear plate 22), whereby the rotation speed of front plate 21 is reduced despite the normal combustion. As a result, the rotation fluctuation exceeds a threshold value, and detection is made as if a misfire (for example, a misfire in the sixth cylinder #6) has occurred.
As described above, an engine employing a flywheel damper has the problem that a misfire is erroneously detected in the idling operation range (the low load/low rotation speed range: range A in FIG. 4). In the drive range of intermediate-load (range C in FIG. 4) also, there are the state where rear plate 22 contacts spring 23 and the state where rear plate 22 does not contact spring 23 (the range of arrow C in FIG. 7), and therefore the rotation fluctuation may become irregular when a misfire has occurred, and thus the misfire may erroneously be detected.
Japanese Patent Laying-Open Nos. 2004-324524 and 07-279734 do not consider the problems associated with use of a flywheel damper as described above. Therefore, the techniques disclosed in the publications cannot solve the problems of rotation fluctuation due to a flywheel damper.